Clean up and deduplicate manifests throughout the repo

[olivi-eh]

Currently, base Kubernetes manifests for Online Boutique live in 4 distinct places:

• kubernetes-manifests/
• kustomize/base/
• release/kubernetes-manifests.yaml
• helm-chart/templates/

And Istio manifests live in 3 distinct places:

• kustomize/components/service-mesh-istio
• istio-manifests/
• release/istio-manifests.yaml

This makes releasing, but also figuring out which manifests are source of truths and which are generated copies, needlessly difficult.

At a minimum, we should (more-or-less in this order):

• Discontinue the use of istio-manifests/ in docs / tutorials (in favour of the Kustomize component), and remove this directory.
• Discontinue the use of release/istio-manifests.yaml in docs / tutorials (in favour of the Kustomize component), and remove this file.
• Improve consistency between releases of the Kubernetes manifests and helm-chart/templates/ (through README notes and new PR requirements).
• Discontinue the use of kubernetes-manifests/ in docs / tutorials (in favour of kustomize/base/ which should be latest upstream and not latest release), and remove this directory.

Additionally, or later, we could also consider:

• Modifying docs / tutorials to use a specific release tag (e.g. v0) instead of cloning latest, and using the kustomize/ directory at that slice in time.
• Discontinue the use of release/kubernetes-manifests.yaml (in favour of the slice in time of kustomize/), and remove this directory.